The influence of low back pain on muscle activity and coordination during gait: a clinical and experimental study

The effect of musculoskeletal pain on motor activity and control

Aberrant movement patterns and postures are obvious to clinicians managing patients with musculoskeletal pain. However, some changes in motor function that occur in the presence of pain are less apparent. Clinical and basic science investigations have provided evidence of the effects of nociception on aspects of motor function. Both increases and decreases in muscle activity have been shown, along with alterations in neuronal control mechanisms, proprioception, and local muscle morphology. Various models have been proposed in an attempt to provide an explanation for some of these changes. These include the vicious cycle and pain adaptation models. Recent research has seen the emergence of a new model in which patterns of muscle activation and recruitment are altered in the presence of pain (neuromuscular activation model). These changes seem to particularly affect the ability of muscles to perform synergistic functions related to maintaining joint stability and control. These changes are believed to persist into the period of chronicity. This review shows current knowledge of the effect of musculoskeletal pain on the motor system and presents the various proposed models, in addition to other shown effects not covered by these models. The relevance of these models to both acute and chronic pain is considered. It is apparent that people experiencing musculoskeletal pain exhibit complex motor responses that may show some variation with the time course of the disorder.

Excitatory actions of experimental muscle pain on early and late components of human jaw stretch reflexes

It has recently been shown that a slow stretch evokes a short-latency (probably monosynaptic) and a long-latency (polysynaptic) reflex response in human jaw-closing muscles. The effect of nociceptive muscle input on the fusimotor system has not been investigated in detail. In order to investigate the effect of sustained muscle pain on the jaw stretch reflex, two main experiments were performed. Stretch reflex responses were evoked in the masseter and temporalis muscles by slow stretches (1-mm displacement, 40-ms ramp time) before, during and 15 min after a period of experimentally induced muscle pain. In experiment I, a dose of 1.0 M hypertonic or 154 mM isotonic (control) saline was infused in random order into the left masseter for up to 15 min (n=12). The level of excitation of the left masseter at 15% maximal voluntary contraction was controlled by visual feedback of the surface EMG (sEMG). In experiment II, a dose of 1.0 M saline was infused into the left masseter but with feedback from the sEMG of the right masseter (n=12). In a control experiment, both sEMG and intramuscular EMG (imEMG) were recorded from the left and right masseters; the feedback was from imEMG of the left masseter (n=12). The early (onset: 9--10 ms) and late (duration from 25 to 40 ms) reflex components were recorded and analysed in all experiments. Infusion of 1.0 M saline caused moderate pain (mean score on a Visual Analogue Pain Scale: 4.9--5.0 cm). The peak-to-peak amplitude of the early reflex component in the painful masseter normalized to the pre-stimulus EMG activity was significantly higher during the pain than the pre- and post-infusion conditions in all experiments. The normalized area of the late reflex component in the painful masseter was significantly larger than in the pre-infusion condition in all experiments. Isotonic saline had no significant effect on the jaw stretch reflexes. These results indicate that experimental jaw-muscle pain in humans facilitates the early as well as the late component of the jaw stretch reflex response as revealed by both sEMG and imEMG. This effect appears to be independent of the level of excitation of the muscle and not related to volume effects of the injected saline. A change in the sensitivity of the fusimotor system during muscle pain is suggested as an explanation.

Multifidus EMG and tension-relaxation recovery after prolonged static lumbar flexion

STUDY DESIGN

The electromyogram (EMG) from the in vivo feline L1 to the L7 multifidus was recorded during the application of a 20-minute static lumbar flexion and after 7 hours of rest.

OBJECTIVE

To determine the recovery of tension-relaxation and laxity in the lumbar viscoelastic structures as well as the recovery of reflexive EMG activity in the multifidus after prolonged static flexion.

SUMMARY OF BACKGROUND

It has been established that prolonged static flexion of the spine induces creep or tension-relaxation in its viscoelastic structures as well as a sharp decrease in the reflexive activity of the dorsal musculature and initiation of spasms. Epidemiologic studies have pointed out that such static flexion is associated with unusually high rates of low back disorders. The rate and pattern of recovery of reflexive muscular activity with rest after static flexion is still unknown.

METHODS

The lumbar spines of seven in vivo feline preparations were subjected to 20 minutes of passive anterior flexion followed by 7 hours of rest while monitoring flexion tension, EMG from the L1-L7 multifidus muscles, and the strain of the L4/L5 supraspinal ligament. A model describing the pattern of recovery of muscular activity and viscoelastic tension was developed.

RESULTS

Twenty minutes of lumbar flexion was associated with an initial sharp decrease of multifidus EMG activity followed by spasms. During rest, EMG activity demonstrated an initial hyperexcitability on flexion, followed by an exponential recovery of muscle activity. Full recovery of residual strain in the L4/L5 supraspinous ligament and multifidus activity was not obtained after 7 hours of rest.

CONCLUSIONS

Static flexion of the lumbar spine is an extremely imposing function on its viscoelastic tissues, resulting in spasms and requiring long periods of rest before normal functions are re-established.

Disorders in trunk rotation during walking in patients with low back pain: a dynamical systems approach

OBJECTIVE

(1) To introduce an evaluation tool for the assessment of walking disorders in low back pain patients. (2) To investigate whether walking patterns in low back pain patients are different from those of control subjects.

DESIGN

Relative phase measures of movement coordination are applied in the assessment of trunk function in a small group of patients with non-specific low back pain and in control subjects.

BACKGROUND

Normal subjects change the coordination of pelvic and thoracic rotations from an in-phase to an out-of-phase pattern with increasing walking speed. Low back pain patients may have a reduced ability to counter rotate pelvis and thorax at higher walking speeds (from 1.0 m/s onwards) as a result of hyperstable coordination patterns.

METHODS

Six patients with non-specific low back pain and six healthy control subjects walked on a treadmill at comfortable walking speeds and during a systematic variation of the treadmill velocity. Coordination of arm and leg movements as well as of pelvic and thoracic rotations was analyzed using a relative phase algorithm.

RESULTS AND CONCLUSIONS

The comfortable walking speed was reduced in the patient group. In contrast to the control subjects, four of the six patients were not able to establish an out-of-phase coordination pattern between thorax and pelvis at higher walking speeds. This coincided with an increased stability of movement coordination, indicating guarded behavior. In addition, an increased asymmetry between the phase-relations of left and right side of the body was found in some of the patients.

Abnormalities of the soleus H-reflex in lumbar spondylolisthesis: a possible early sign of bilateral S1 root dysfunction

Using routine electrodiagnostic procedures, the authors searched for physiologic evidence of nerve root compromise in patients with chronic mechanical perturbation to the lumbar spine. They examined 37 patients with spondylolisthesis and various degrees of degenerative changes in the lumbar canal. Clinical and neurophysiologic findings were compared with data obtained from 36 healthy persons. The soleus H-reflex appeared to be a sensitive indicator of sensory fiber compromise at the S1 root level, because changes correlated well with the focal sensory signs and preceded clinical and electromyographic signs of motor root involvement. When these occurred, the clinical findings were consistent with a more severe nerve root deficit and with radiographic evidence of neural compression. The greater sensitivity of the soleus H-reflex may be related to the pathophysiologic events that occur at the lesion site.

Effect of experimental pain from trigeminal muscle and skin on motor cortex excitability in humans

The pathophysiology of many orofacial pain syndromes is still unclear. We investigated the effect of tonic muscle and skin pain on the excitability of the trigeminal motor pathways using transcranial magnetic stimulation (TMS). Motor evoked potentials (MEPs) were recorded in the masseter surface electromyogram (EMG). Magnetic pulses were delivered with a large coil at intensities 1.1 and 1.5 times the motor threshold, and for each intensity, MEPs were recorded at three different clenching levels: 15, 30 and 45% of maximum voluntary contraction (MVC). Baseline, pain and post-baseline recordings were compared in two sessions. Firstly, muscle pain was induced by infusion of hypertonic saline (5.8%) into the left masseter. Secondly, skin pain was induced by topical application of capsaicin (5%) on the left cheek. Muscle and skin pain did not induce significant effects on the amplitude or latency of the MEPs (ANOVAs: P>0.50). In both sessions, the amplitude of the MEPs increased with the increase of the clenching level and stimulus intensity (P<0.0001; P<0.005) whereas the latency was not significantly changed (P>0.05; P=0.11). Muscle pain was associated with an increase in the pre-stimulus EMG activity on the non-painful side compared with baseline (P<0.01), which could be due to compensatory changes in the activation of the painful muscle. The need for voluntary contraction to evoke MEPs in the masseter muscles and compensatory mechanisms both at the brainstem and cortical level might explain the lack of detectable modulation of MEPs. Nonetheless, the present findings did not support the so-called 'vicious cycle' between pain - central hyperexcitability - muscle hyperactivity.

Effect of muscle pain and intrathecal AP-5 on electromyographic patterns during treadmill walking in the rat

1. A physio-neuropharmacological model to assess in the rat the contribution of muscular nociceptive input to motor dysfunction and pharmacological aspects of spinal sensorimotor interaction, during treadmill walking, is presented. 2. Rats were trained to walk on a treadmill prior to chronic electromyographic electrode implantation and intrathecal catheterization. 3. Changes in electromyographic activities were recorded from the median gastrocnemius and tibialis anterior muscles of the right hindlimb, during rhythmic locomotor movements. 4. Intramuscular hypertonic saline 6% was injected into the right triceps surae muscles to produce an experimental muscle pain during treadmill walking. 5. Pain produced a decrease in both median gastrocnemius and tibialis anterior electromyographic activities during rhythmic locomotion. Mean gastrocnemius burst duration decreased while tibialis anterior burst duration increased. 6. AP-5, an NMDA receptor antagonist known to block the synaptic excitatory drive to central pattern generators, was injected intrathecally at the level of the lumbar spinal cord to validate this neuropharmacological model. 7. Intrathecal AP-5 induced a decrease of both median gastrocnemius and tibialis anterior muscle activities during treadmill walking. This pharmacological intervention aggravated behavioural and motor effects of muscle pain.

The role of the motor system in spinal pain: implications for rehabilitation of the athlete following lower back pain

The purpose of this review is to consider the role of the motor system in spinal pain. It is well accepted that spinal stability is dependent on the contribution of the muscular system. However, the ability of this system to satisfy the requirements of stability is dependent on its controller--the central nervous system (CNS). The CNS must predict the outcome of movements to plan appropriate strategies of muscle activity to meet the demands of internal and external forces, and initiate appropriate responses to unexpected disturbances. In addition, this complex control of stability must occur in conjunction with control of the trunk muscles for other functions, such as respiration. For the CNS to cope with athletic performance the coordination of these parameters must be streamlined. Yet evidence suggests that when spinal pain is present the strategies used by the CNS to control trunk muscles may be altered. The mechanism for these changes is poorly understood but may be due to changes at many levels of the CNS. For rehabilitation of the athlete with spinal pain it is critical that the motor control of stability is optimised. Furthermore, this must be coordinated with the multiple other functions of trunk muscles, including respiration.

Fear-avoidance and its consequences in chronic musculoskeletal pain: a state of the art

In an attempt to explain how and why some individuals with musculoskeletal pain develop a chronic pain syndrome, Lethem et al. (Lethem J, Slade PD, Troup JDG, Bentley G. Outline of fear-avoidance model of exaggerated pain perceptions. Behav Res Ther 1983; 21: 401-408).ntroduced a so-called 'fear-avoidance' model. The central concept of their model is fear of pain. 'Confrontation' and 'avoidance' are postulated as the two extreme responses to this fear, of which the former leads to the reduction of fear over time. The latter, however, leads to the maintenance or exacerbation of fear, possibly generating a phobic state. In the last decade, an increasing number of investigations have corroborated and refined the fear-avoidance model. The aim of this paper is to review the existing evidence for the mediating role of pain-related fear, and its immediate and long-term consequences in the initiation and maintenance of chronic pain disability. We first highlight possible precursors of pain-related fear including the role negative appraisal of internal and external stimuli, negative affectivity and anxiety sensitivity may play. Subsequently, a number of fear-related processes will be discussed including escape and avoidance behaviors resulting in poor behavioral performance, hypervigilance to internal and external illness information, muscular reactivity, and physical disuse in terms of deconditioning and guarded movement. We also review the available assessment methods for the quantification of pain-related fear and avoidance. Finally, we discuss the implications of the recent findings for the prevention and treatment of chronic musculoskeletal pain. Although there are still a number of unresolved issues which merit future research attention, pain-related fear and avoidance appear to be an essential feature of the development of a chronic problem for a substantial number of patients with musculoskeletal pain.

Muscle activation in the contralateral passive shoulder during isometric shoulder abduction in patients with unilateral shoulder pain

Studies have shown an increased muscle activation at the opposite passive side during unilateral contractions. The purpose of the present study was to examine the influence of pain on muscle activation in the passive shoulder during unilateral shoulder abduction. Ten patients with unilateral rotator tendinosis of the shoulder and nine healthy controls performed unilateral maximal voluntary contractions (MVC) and sustained submaximal contractions with and without subacromial injections of local anaesthetics of the afflicted shoulder. Muscle activation was recorded by electromyography (EMG) from the trapezius, deltoid, infraspinatus and supraspinatus muscles in both shoulders. During MVCs, the EMG amplitude from muscles of the passive afflicted side was not different in patients and controls, and was not influenced by pain alterations. In contrast, the EMG amplitude from the muscles of the passive unafflicted side was lower in the patients and increased after pain reduction. During the sustained submaximal contraction the EMG amplitude increased gradually in the passive shoulder to 15-30% of the EMG amplitude observed during MVC. This response was not influenced by differences in pain. We conclude that muscle activation of the passive shoulder was closely related to the activation of the contracting muscles and thus related to central motor drive, and not directly influenced by changes in pain.

Biexponential recovery model of lumbar viscoelastic laxity and reflexive muscular activity after prolonged cyclic loading

OBJECTIVES

To determine the rest duration required for full recovery of reflexive muscular activity and laxity/creep induced in the lumbar viscoelastic structures (e.g., ligaments, discs, etc.) after 50 min of cyclic loading, and to develop a model describing such recovery.

BACKGROUND

It is well established that steady, cyclic or vibratory loading of the lumbar spine induces laxity/creep in its viscoelastic structures. It was also shown that such viscoelastic creep does not fully recover when subjected to rest equal in duration to the loading period. Rest periods of 24 h, however, were more than sufficient to allow full recovery. The exact period of time allowing full recovery of viscoelastic laxity/creep, and its pattern is not known. It is also not known what is the duration required for full recovery of reflexive muscular activity lost due to the laxity/creep induced in the spine during cyclic loading.

METHODS

The lumbar spine of 'in vivo' feline preparations was subjected to 50 min of 0.25 Hz cyclic loading applied v ia the L4/5 supraspinal ligament. At the end of the loading period the spine was subjected to prolonged rest, interrupted by a single cycle loading applied hourly for measurement purposes until the laxity was fully recovered (>90%). Reflexive EMG activity was recorded with wire electrodes from the L-1-L-7 multifidus muscles. A biexponential model was fitted to the load and EMG recorded in the recovery period in order to represent viscous and elastic components of structures with different architecture (e.g., disc vs. ligament).

RESULTS

Full recovery of the laxity induced by 50 min of cyclic loading at 0.25 Hz required 7 h and was successfully fitted with a biexponential model. Similarly, EMG activity was fully recovered in 4 hours, and often exceeded its initial value during the following 3 h.

CONCLUSIONS

Full recovery of laxity induced in the lumbar viscoelastic structures by a given period of cyclic loading requires rest periods, which are several folds longer than the loading duration. Similarly, reflexive muscular activity requires 4 h of rest in order to be restored. Meanwhile, significant laxity can be present in the joints, exposing the spine to potential injury and low back pain. Increased EMG activity at the end of the recovery period may indicate that pain was possibly induced in the spinal structures, inducing hyperexcitability of the muscles during passive loading.

RELEVANCE

Although the data was derived from a feline model, and its extrapolation to the human model is not straightforward, the general pattern of decreasing reflexive muscular activity with cyclic loading is expected in both species. Therefore, workers who subject their spine to periods of cyclic loading may be exposed to prolonged periods of laxity beyond the neutral zone limits, without protection from the muscles and therefore the risk of possible injury and low back pain. Pain and muscle hyperexcitability could also be a factor associated with cyclic loading, being expressed several hours after work was completed.

Dynamic adjustments of walking behavior dependent on noxious input in experimental low back pain

The aim of this study was to explore whether accelerations of the lower back during walking are temporarily attenuated by experimentally-induced low back pain, as compared with normal walking. Transient low back pain was induced by injection of 1 ml 6% hypertonic saline in the longissimus dorsi muscle in 20 healthy subjects. Acceleration was measured during walking at self-selected speeds before and repeatedly after the injection by a portable, triaxial accelerometer positioned over the L3 region. Data were subsequently adjusted for differences in walking speeds between trials and subjects. Pain was reported on a 0-10 point scale during walking until pain was no longer present. Lumbar acceleration sample mean was attenuated for the anteroposterior (P=0.002) and mediolateral (P=0.002) sensing axes as well as for the vector sum (P=0.005) at maximal pain compared to pretest values. The vertical axis showed no significant changes. Values returned to pretest level when pain was no longer present. Regardless of the initial increase and subsequent decrease in pain after injection, there was a linear relationship between pain and acceleration in 15 of the 20 subjects (0.89>/=R(2)>/=0.36, P</=0.002), suggesting a continuous dynamic adjustment of motor behavior dependent on the noxious input. It appears that this new method is suitable for detection of continuous subconscious changes of body accelerations during walking in relation to pain.

Reorganisation of human step initiation during acute experimental muscle pain

The effects of experimentally induced pain in tibialis anterior on step initiation were studied in six healthy male volunteers by kinematics, kinetics and electromyography (EMG). We observed longer reaction times, lower joint moments and decreased EMG activity in the painful leading leg's tibialis anterior and gastrocnemius muscles. Peripheral nociceptive input appeared to alter the internal representation used in the step initiation motor program.

Recent concepts in the neurophysiology of pain

This paper describes many of the processes that exist for upregulation of the nociceptive system in response to tissue injury. The processes of peripheral and central sensitization are described. Potential interactions between the nociceptive, motor and autonomic systems are considered. The potential for psychosocial factors to influence neuroplasticity within the nociceptive system is also discussed.

Voluntary and reflex control of human back muscles during induced pain

1. Back pain is known to change motor patterns of the trunk. The purpose of this study was to examine the motor output of the erector spinae (ES) muscles during pain in the lumbar region. First, their voluntary activation was assessed during flexion and re-extension of the trunk. Second, effects of cutaneous and muscle pain on the ES stretch reflex were measured, since increased stretch reflex gain has been suggested to underlie increased muscle tone in painful muscles. 2. The trunk movement and electromyographical (EMG) signals from the right and left ES during pain were compared with values before pain. Controlled muscle pain was induced by infusion of 5 % saline into the right lumbar ES. Cutaneous pain was elicited by mechanical or electrical stimulation of the dorsal lumbar skin. The stretch reflex was evoked by rapidly indenting the right lumbar ES with a servo-motor prodder. 3. The results from the voluntary task show that muscle pain decreased the modulation depth of ES EMG activity. This pattern was associated with a decreased range and velocity of motion of the painful body segment, which would normally serve to avoid further injury. Interestingly, when subjects overcame this guarding tendency and made exactly the same movements during pain as before pain, the EMG modulation depth was still reduced. The results seem to reconcile the controversy of previous studies, in which both hyper- and hypoactivity of back muscles in pain have been reported. 4. In the tapped muscle, the EMG response consisted of two peaks (latency 19.3 +/- 2.1 and 44.6 +/- 2.5 ms, respectively) followed by a trough. On the contralateral side the first response was a trough (26.2 +/- 3.2 ms) while the second (46.4 +/- 4.3 ms) was a peak, similar to the second peak on the tapped side. Cutaneous pain had no effect on the short-latency response but significantly increased the second response on the tapped side. Surprisingly, deep muscle pain had no effect on the stretch reflex. A short-latency reciprocal inhibition exists between the right and left human ES. 5. It is concluded that deep back pain does not influence the stretch reflexes in the back muscles but modulates the voluntary activation of these muscles.

Evidence for subnucleus interpolaris in craniofacial muscle pain mechanisms demonstrated by intramuscular injections with hypertonic saline

The subnucleus interpolaris (Vi) has been identified as a major recipient for trigeminal ganglionic input from jaw muscles, and contains neurons with nociceptive properties similar to those in the subnucleus caudalis (Vc). Therefore, Vi may be another important site for processing craniofacial muscle nociception. The aims of present study were to define functional properties of Vi neurons that receive input from masseter muscle afferents by characterizing their responses to electrical, mechanical, and to chemical stimulation of the muscle. Ninety cells were identified as masseter muscle units in 11 adult cats. Most of these units (79%) received additional inputs from orofacial skin. Following the intramuscular injection of 5% hypertonic saline, 49% of the cells showed a significant modulation of either the resting discharge and/or responses to innocuous mechanical stimulation on their cutaneous receptive fields (RFs). The most common response to saline injection was an induction or facilitation of resting discharge which declined as an exponential decay function, returning to pre-injection level within 3-4 min. Forty-five percent of the muscle units that were tested with mechanical stimulation (13/29) showed a prolonged inhibition of mechanically-evoked responses. In most cases (8/13), the inhibitory response was accompanied by initial facilitation. The observations that Vi contained a population of neurons that receive small diameter muscle afferent inputs, responded to noxious mechanical stimulation on the muscle and to a chemical irritant that is known to produce pain in humans provide compelling evidence for the involvement of Vi in craniofacial muscle pain mechanisms.

Brainstem mechanisms underlying feeding behaviors

The essential elements controlling trigeminal motoneurons during feeding lie between the trigeminal and facial motor nuclei. These include populations of neurons in the medial reticular formation and pre-motoneurons in the lateral brainstem that reorganize to generate various patterns. Orofacial sensory feedback, antidromic firing in spindle afferents and intrinsic properties of motoneurons also contribute to the final masticatory motor output.

Experimental muscle pain increases the human stretch reflex

In this study we investigated the effect of human experimental muscle pain on H- and stretch reflexes as indicators of changes in muscle spindle sensitivity. Fourteen healthy, male volunteers participated in the study. Muscle pain was produced by infusion of 5% hypertonic saline over a period of 10-15 min in m. soleus and in m. tibialis anterior. Reflexes were elicited in the relaxed and active soleus muscle (10-15 Nm ankle torque) before, during and after muscle pain. Control measurements were made with infusions of 0.9% isotonic saline. Surface electromyograms (EMG) were measured from the soleus muscle, and torque was measured from the ankle joint. With pain in the soleus muscle the mechanical stretch reflex response (ankle torque) increased significantly (P = 0.0007) as compared to before pain. With pain in the tibialis anterior muscle both the mechanical and EMG responses increased significantly (P = 0.001; P = 0.0003) as compared to before pain. The H-reflex showed no significant changes during the infusions in either muscles. This study has demonstrated a muscle pain-related increase in the amplitude of the stretch reflex without a corresponding increase in the H-reflex amplitude. One explanation could be an increased dynamic sensitivity of the muscle spindles during muscle pain caused by an increased firing rate in the dynamic gamma-motoneurones. However, the data could not support the vicious cycle model because the excitability of the alpha-motoneurone pool was unchanged.

Effects of chemical stimulation of masseter muscle nociceptors on trigeminal motoneuron and interneuron activities during fictive mastication in the rabbit

An electrophysiological study was carried out in sixteen decerebrate and paralyzed New Zealand rabbits to determine how a bolus injection of a nociceptor stimulant (hypertonic saline, 5%) into the masseter muscle influences the activity of the trigeminal motor circuitry during fictive jaw movements. Hypodermic needles connected to a syringe held in a computer-controlled infusion pump were inserted into the anterior deep layer of either the right or the left masseter. Twenty-three infusions of 50, 70 or 80 microl saline were made in fourteen animals at constant rates over 1 min. Eight control infusions of normal saline (0.9%) were made in a subpopulation of five animals in an identical manner. Fictive jaw movements were evoked before and after the infusions by repetitive electrical stimulation of the corticobulbar tract. Effects were assessed by extracellular microelectrode recordings made from the digastric motoneuron pool and from putative last-order interneurons in the oral subnucleus of the spinal trigeminal tract and adjacent structures. In comparison with pre-infusion control cycles, nociceptor stimulation caused significant slowing of the rhythm and a reduction of the area of the digastric motoneuron bursts in the majority of the animals (12/14). The decrease in cycle frequency was due almost entirely to a lengthening of the time between the digastric bursts. Changes usually began 1-2 min after the infusion and returned to pre-infusion values within 10-15 min. No significant effects were seen when isotonic saline was applied. Recordings were obtained from nine interneurons, eight of which had low threshold mechanosensitive receptive fields. One neuron was, in addition, excited by pinch. Eight were not active in the absence of motor activity and this did not change when hypertonic saline was applied. However, once fictive movements began, all started to fire rhythmic bursts of spikes. In five cases, there was a significant post-infusion increase in spike frequency, and three showed decreases. Seven showed significant post-infusion changes in mean phase and/or concentration of their firing within the movement cycle. Changes in the preferred phase of interneuronal firing were significantly correlated to changes in the phase of offset of the digastric burst. The present results provide evidence that the stimulation of nociceptors in a muscle slows the frequency of rhythmical movements in the absence of sensory feedback. They confirm that infusions into one muscle affect the output of its antagonist. The results also suggest that neurons in the oral subnucleus of the spinal trigeminal tract and adjacent reticular formation appear to participate in programming these changes in motor output.

Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function

The relation between muscle pain, muscle activity, and muscle co-ordination is still controversial. The present human study investigates the influence of experimental muscle pain on resting, static, and dynamic muscle activity. In the resting and static experiments, the electromyography (EMG) activity and the contraction force of m. tibialis anterior were assessed before and after injection of 0.5 ml hypertonic saline (5%) into the same muscle. In the dynamic experiment, injections of 0.5 ml hypertonic saline (5%) were performed into either m. tibialis anterior (TA) or m. gastrocnemius (GA) and the muscle activity and co-ordination were investigated during gait on a treadmill by EMG recordings from m. TA and m. GA. At rest no evidence of EMG hyperactivity was found during muscle pain. The maximal voluntary contraction (MVC) during muscle pain was significantly lower than the control condition (P < 0.05). During a static contraction at 80% of the pre-pain MVC muscle pain caused a significant reduction in endurance time (P < 0.043). During dynamic contractions, muscle pain resulted in a significant decrease of the EMG activity in the muscle, agonistic to the painful muscle (P < 0.05), and a significant increase of the EMG activity of the muscle, antagonistic to the painful muscle (P < 0.05). Muscle pain seems to cause a general protection of painful muscles during both static and dynamic contractions. The increased EMG activity of the muscle antagonistic to the painful muscle is probably a functional adaptation of muscle co-ordination in order to limit movements. Modulation of muscle activity by muscle pain could be controlled via inhibition of muscles agonistic to the movement and/or excitation of muscles antagonistic to the movement. The present results are in accordance with the pain-adaptation model (Lund, J.P., Stohler, C.S. and Widmer, C.G. In: H. Vaerøy and H. Merskey (Eds.), Progress in Fibromyalgia and Myofascial Pain. Elsevier, Amsterdam, 1993, pp. 311-327.) which predicts increased activity of antagonistic muscle and decreased activity of agonistic muscle during experimental and clinical muscle pain.

Sensory-motor interactions of human experimental unilateral jaw muscle pain: a quantitative analysis

Experimental muscle pain was elicited by bolus injection of 0.15 ml of 5% hypertonic saline into the human masseter muscle. The sensory experience was described using 10-cm visual analogue scales (VAS) and the McGill Pain Questionnaires (MPQ) on 10 subjects. Effects of pain on deliberately unilateral mastication were quantitatively assessed in 13 other male subjects using kinematic recordings of the mandible and jaw muscle electromyography (EMG). Jaw movement and EMG data were transformed into single masticatory cycles which were averaged within subjects to produce mean masticatory cycles. Injection of 5% saline through normal and anesthetized skin produced similar VAS profiles and MPQ features. Displacement of the mandible during painful mastication was significantly smaller in the vertical axis (10.0 +/- 11.5%, P < 0.05) and in the lateral axis (22.6 +/- 20.9%, P < 0.05) as compared to pre-pain values. The mean opening and closing velocities of the mandible were significantly reduced (10.5 +/- 16.3% and 15.3 +/- 21.2%, P < 0.05) and the cumulated distance of the jaw movement was also significantly smaller during pain (10.5 +/- 11.8%, P < 0.05). Moreover, agonist EMG activity during pain was significantly lower in the ipsilateral masseter muscle (20.3 +/- 25.4%, P < 0.05) as compared to pre-pain root-mean-square (RMS) values. The observed sensory-motor interactions can be explained by a facilitatory effect of activity in nociceptive muscle afferents on inhibitory brain-stem interneurons during agonist action. Thus, generated movements have smaller amplitudes and they are slower which most likely represents a functional adaptation to experimental jaw muscle pain.